Titanium chemical milling etchant

ABSTRACT

A composition and a process for chemical milling titanium utilizing an aqueous nitric-hydrofluoric acid base etchant having a nitric acid content ranging from about 0.2 - 1.2 weight percent.

United States Patent 1 Roni [ TITANIUM CHEMICAL MILLING ETCHANT [75]Inventor: Jack C. Roni, Seattle, Wash.

[73] Assignee: The Boeing Company, Seattle,

Wash.

[22] Filed: June 20, 1972 [21] Appl. No.: 264,594

Related US. Application Data [63] Continuation of Ser. No. 885,598, Dec.16, 1969,

abandoned.

[52] US. Cl 156/18, 252/79.3, 252/79.4 [51] Int. Cl. C231 1/00 [58]Field of Search 252/79.3, 79.4; 156/18,

[ Get. 29, 1974 [56] References Cited UNlTED STATES PATENTS 2,981,6094/1961 Acker et al. 252/79.3 2,981,610 4/1961 Snyder et a1 252/79.33,048,503 8/1962 Foote et a1 156/18 X 3,598,741 8/1971 Shozo Kanno252/79.3

Primary ExaminerWilliam A. Powell Attorney, Agent, or Firm-Morris A.Case; Glenn Orlob 5 7 ABSTRACT A composition and a process for chemicalmilling titanium utilizing an aqueous nitric-hydrofluoric acid baseetchant having a nitric acid content ranging from about 0.2 1.2 weightpercent.

2 Claims, 1 Drawing Figure Amie/2' A00 [aware/Mam, n f

TITANIUM CHEMICAL MILLING ETCIIANT This is a streamlined continuation ofprior copending application, Ser. No. 885,598. filed Dec. 16, 1969, andsince abandoned.

BACKGROUND OF THE INVENTION Commercially used titanium chemical millingsolutions are generally of a nitric-hydrofluoric acid solution or of achromic-hydrofluoric acid solution. The nitrichydrofluoric acid solutionis most often used as it has a longer work life, lower material cost, iseasier to control, has less smut block problems, and gives about thesame milling characteristics.

Milling characteristics of these etchants leave much to be desired. Thenitric-hydrofluoric acid etchant has a non-uniform rate of metalremoval. Formed parts having both horizontal and vertical surfacesundergo a faster rate of metal removal on the vertical surfaces. Thisetch rate differential may vary by as much as 40 percent. In addition,considerable taper is produced on vertical surfaces, with surface areashigher in the etching tank having the metal removed faster than surfaceareas lower in the tank. A transition zone, which is defined as thedistance from the edge of a milled cut to the point where nominalthickness is reached, is about ten times the depth of the cut. Channelsand ridges are formed in this transition zone at the base of theuppermost fillets of vertically milled areas, and a fillet overhang iscreated at the edge of the cut. The ridges and the overhang must bemechanically removed after completion of the chemical milling.

It has become common practice to partially compensate for non-uniformmilling characteristics by periodically removing the part from theetchant and turning the part down side up before re-immersing.

It was discovered that uniform milling characteristics can be obtained,the transition zone can be reduced to about three times the depth ofcut, and mechanical removal after chemical milling becomes unnecessaryby using a nitric-hydrofluoric acid solution or etchant bath wherein thenitric acid content is reduced to a concentration of about 0.2 1.2weight percent. Ammonium bifluoride may be added to further reduce or tocompletely remove channeling and ridging. It was further discoverednitric acid in concentrations as low as 0.2 percent by weight will stilllimit hydrogen absorption by titanium. Below that concentrationexcessive hydrogen absorption does take place.

An object of this invention is to obtain uniform chemical millingcharacteristics for titanium.

Another object of this invention is to obtain uniform chemical millingcharacteristics on all surfaces of titanium parts having both horizontaland vertical surfaces.

Another object of this invention is to chemical mill titanium withoutimparting a taper on vertical surfaces.

Still another object is to reduce the length of the transition zone whenchemical milling titanium.

Yet another object is to chemical mill titanium without leavingextraneous metal or creating a smut block which must be removed bymechanical means.

DESCRIPTION OF DRAWING The FIGURE is of a graph depicting optimum nitricacid concentration versus concentration of dissolved titanium in thedisclosed chemical etchant.

DETAILED DESCRIPTION The nitric acid content of an aqueousnitrichydrofluoric acid solution or etchant was discovered ascontrolling the uniformity of removal of titanium during chemicalmilling. Tests were performed using a solution containing nitric acidand sufficient hydrofluoric acid to effect an etch rate of 0.0008 to0.0010 inches per minute per side, and dodecylbenzene sulfonic acid as asurface active agent in quantities sufficient to control surface tensionof the solution at 30 35 dynes per cm. Formed titanium parts havinghorizontal and vertical surfaces were immersed in the etchant for 40minutes with the etchant bath at a temperature of 1 15F 3F. A series oftests were performed in which the nitric acid concentration was varied.Table I below shows the effect of varying the nitric acid concentration.

Table I Nitric Acid Concentration Percent difference refers to rate ofmetal removal on a horizontal surface with respect to rate of removalfrom a vertical surface.

A negative figure is obtained when metal is removed from a horizontalsurface faster than from a vertical surface.

Conversely a positive figure is obtained when metal is removed from avertical surface faster than from a horizontal surface.

The percent difference is represented by the equation:

Percent Difference (vertical surface-horizonal surface)/(verticalsurface) When titanium is removed it goes into solution in the etchantbath, and will remain in solution until the titanium concentrationreaches about 8.7 weight percent at which point it starts to precipitateout. As the dissolved titanium concentration builds up the hydrofluoricacid and the nitric acid concentration must be increased to maintainrate of metal removal and uniformity of removal. In Table I, above, thedissolved titanium concentration ranged from about 1.8 to 2.2 weightpercent. Table II shows the effect of varying nitric acid concentrationas the dissolved titanium concentration varies.

Table II Dissolved Titanium Nitric Acid Weight Percent DissolvedTitanium Weight Percent Percent Difference 4.9 0.64 +l.9 6.4 0.82 --3.06.9 L08 -l.0

dissolved titanium concentration to effect uniform rate of metalremoval, reduce the transition zone, and to obtain etching which doesnot require mechanical removal of metal after chemical milling. Abovethe optimum range, area 2, metal is removed from a vertical surfacefaster than from a horizontal surface and a taper is produced on avertical surface with the metal being removed more rapidly on thesurfaces which are located higher in the chemical milling bath. Belowthe optimum range, area 3, the metal removal characteristics areslightly reversed. The horizontal surface is milled faster than thevertical surface and a taper is produced on a vertical surface with themetal being re moved more rapidly at the lower part of the surface inthe chemical mill bath. Difficulties may also be experienced below about0.20 weight percent nitric acid due to excessive hydrogen ionabsorption. Above about 8.7 weight percent of dissolved titanium thetitanium precipitates out as a fluoride.

A preferred chemical milling solution or etchant may be 0.2 1.2 weightpercent nitric acid, 4 18 weight percent total fluoride and a surfaceactive agent in amounts sufficient to control surface tension at from 2860 dynes per cm. with about 33 dynes per cm. preferred.

ln another embodiment the hydrofluoric acid concentration may besufficient to effect an etch rate of from 0.0004 to 0.0015 inches perside per minute, but with a preferred etch rate of 0.0007 to 0.001 1inches per side per minute The addition of small quantities of ammoniumbifluoride from about 0.07 to 2.9 weight percent will further reduce orcompletely eliminate channeling and ridging in the fillet area.

The surface active agent may be selected from the group consisting ofdodecylbenzene sulfonic acid or linear alkyl sulfonic acid.

A preferred process may use any of the nitrichydrofluoric acid etchantswith a low concentration of nitric acid as herein disclosed and immersetitanium parts in the etchant at from about 90F to 130F but with about F3F preferred, and holding in the etchant bath until the desired depth ofcut is obtained.

In another preferred embodiment for chemical milling titanium utilizinga nitric-hydrofluoric acid etchant the nitric acid concentration may becontrolled at a percentage range such as to effect a uniform rate ofmetal removal on both horizontal and vertical surfaces.

1 claim:

I. A method of chemical milling titanium to remove metal from thehorizontal and vertical surfaces of the titanium at essentially the sameuniform rate, the steps comprising:

a. immersing the titanium in an aqueous etching fluid containing aconcentration of hydrofluoric acid sufficient to effect an etch rate ontitanium of from about 0.0004 to 0.0015 inches per side per minute, asurface active agent selected of materials consisting essentially ofdodecylbenzene sulfonic acid and linear alkyl sulfonic acid in amountssufficient to control surface tension of the etching solution at fromabout 28 to 60 dynes per cm., and about 0.2 weight percent of nitricacid, said aqueous etching fluid uniformly removes titanium metal whichwhen removed goes into solution in the etching fluidf varying the nitricacid concentration in the etchant as the dissolved titaniumconcentration increases such that the nitric acid concentrationincreases from 0.2 to about 1.2 weight percent as the dissolved titaniumweight percent in the solution increases up to about 8.7; and

c. holding the titanium in the etchant until the desired depth of cut isobtained.

2. A method of chemical milling titanium as recited in claim 1, furthercomprising: adding from about 0.07 to 2.9 weight percent of ammoniumbifluoride to the

1. A METHOD OF CHEMICAL MILLING TITANIUM TO REMOVE METAL FROM THE HORIZONTAL AND VERTICAL SURFACES OF THE TITANIUM AT ESSENTIALLY THE SAME UNIFORM RATE, THE STEPS COMPRISING: A. IMMERSING THE TITANIUM IN AN AQUEOUS ETCHING FLUID CONTAINING A CONCENTRATION OF HYDROFLUORIC ACID SUFFICIENT TO EFFECT AN ETCH RATE ON TITANIUM OF FROM ABOUT 0.0004 TO 0.0015 INCHES PER SIDE PER MINUTES, A SURFACE ACTIVE AGENT SELECTED OF MATERIALS CONSISTING ESSENTIALLY OF DODECYLBENZENE SULFONIC ACID AND LINEAR ALKYL SULFONIC ACID IN AMOUNTS SUFFICIENT TO CONTROL SURFACE TENSION OF THE ETCHING SOLUTION AT FROM ABOUT 28 TO 60 DYNES PER CM., AND ABOUT 0.2 WEIGHT PERCENT OF NITRIC ACID, SAID AQUEOUS
 2. A method of chemical milling titanium as recited in claim 1, further comprising: adding from about 0.07 to 2.9 weight percent of ammonium bifluoride to the etchant. 